1. Field of the Invention
The present invention relates to a color image display device, and in particular, to an improvement of brightness uniformity throughout an entire screen in an image display device having a flat-type color cathode ray tube with a high wedge ratio.
2. Description of the Related Art
A conventional color cathode ray tube is a display device generating an image emitting electron beams from an electron gun 10, beams of which are deflected by a deflection yoke 6 and pass through a shadow mask, and arrive at a fluorescent film 7, which is coated with a fluorescent body.
The conventional color cathode ray tube comprises a cathode 3 for inputting video signals, a heater 2 for generating thermal electrons by heating the cathode 3, a grid electrode for controlling the quantity of electron beams by means of a voltage difference from the cathode 3, an electron gun 10 including a triode section, which comprises an accelerating electrode 5 for accelerating the electron beams passed though the grid electrode 4, an 20 anode 9 for applying a high voltage for applying the generated electron beams to a fluorescent body 7, and a deflection yoke 6 for redirecting the electron beams to arrive at the fluorescent body 7 at a desired position.
The cathode ray tube having the above construction is operated as follows. Thermal electrons, which have been generated from the cathode 3 and heated by the heater 2, are accelerated toward the fluorescent body 7 by means of a high voltage on the anode 9 in an amount as proportional to the video signals inputted to the cathode. The fluorescent body 7 becomes luminescent due to collision with the accelerated thermal electrons, and displays an image on a screen. In order to enable the electron beams to arrive at the fluorescent body at a predetermined position, it is critical to generate a magnetic field by flowing a saw wave current from the deflection yoke 6, and allowing the electron beams to pass through the magnetic field and receive a magnetic force (F=xe2x88x92eVxc3x97B).
The quantity of electron beams emanated from the cathode 5 is determined by the voltage difference between the video signal voltage applied to the cathode and the voltage applied to the grid electrode. Assuming that the grid electrode voltage is constant (0V), the beam current becomes inversely-proportional to the cathode voltage.
The conventional cathode ray tube having a curved face has almost the same curve ratio between the inner face and outer face of a screen. Since the inner face of the screen of a panel has a remarkably greater curve ratio than the outer face of the screen, the distance to be covered by the movements of electron beams that have been emitted from the electron gun should be greater around the edge area than around the middle area of the screen. Such differences in distance to be covered by the movements of electron beams pose a problem of deteriorating uniformity in the overall brightness (white balance) due to less brightness around the edge area of the screen than in the middle area of the screen when the electron beams of the same quantity are emitted.
To resolve the above problem, Korean Patent Publication No. 10-0258982 (laid open on Jul. 5, 1999) suggests a monitor, comprising a parabola signal generating section 107 as shown in FIG. 2 for generating a parabola signal, a first video output signal generating section 106 for outputting a first video output signal, which has been adjusted to a brightness adjusting voltage from a black level voltage based on a video input signal, carrying an image with the black level voltage as well as on and input of the brightness adjusting voltage to adjust brightness of the screen, a signal synthesizing section 108 for outputting a second video output signal, which incorporates the parabola signal into the first video output signal based on an input of the first video output signal 106 and the parabola signal, and a brightness adjusting section 101 of the screen.
As shown in FIG. 3, Japanese Laid-Open Patent Publication No. 2000-125225 (laid open on Apr. 28, 2000) also suggests a technology for correcting a brightness level in a pixel unit by means of a function having a predetermined pixel reference position with respect to the video brightness signal as well as a distance of a video display pixel position as factors.
The method for compensating deterioration of the brightness around an edge area is to display the video signals in accordance with positions thereof on the screen. However, it requires a very complicated and expensive brightness compensation circuit to convert the video signals inputted in real time through an antenna. Further, this kind of brightness compensating method is used to resolve the brightness imbalance due to differences in distance to be covered by the movements of electron beams of the cathode ray tube, etc.
With the recent launching of a cathode ray tube having an almost flat outer face of the screen, however, the thickness difference between the middle area and the edge area of the screen has become greater. This weakens the strength of the flat outer face of the screen by differentiating the curve ratios between the inner face and outer face of the screen in the cathode ray tube. This is generally expressed by a wedge ratio. The wedge ratio of a flat-type panel having a flat outer face and a curved inner face ranges up to 230% at the maximum and down to 170% at the minimum. Brightness difference is created with respect to the same signal due to the thickness difference. In an effort to overcome such a brightness difference, a panel is generally manufactured for the screen using clear glass of very high transmissibility. Since the clear glass has a transmissibility of about 80%, a brightness difference is created with respect to the same beam current by the thickness difference in accordance with positions of the screen glass. Thus, efforts have been exerted to obtain uniform brightness through either graded coating or attachment of films with different transmissibility to each position of the screen glass so as to differentiate the transmissibility in accordance with positions thereof on the screen. It is irrational to conclude that uniform brightness cannot be obtained if tint glass with transmissibility of about 50% is used, when the wedge ratio is high, either to elevate quality of the cathode ray tube or to lower the manufacturing cost.
It is, therefore, an object of the present invention to provide a color image display device having an improvement of brightness uniformity throughout an entire screen in an image display device have a flat-type cathode ray tube with a high wedge rate.
To achieve the above object, there is provided a color image display device, comprising: a panel housing a screen on an inner face thereof; a funnel engaged with the panel; an electron gun engaged with a neck portion of the funnel for emitting electron beams toward the screen; a deflection device for displaying an image by deflecting the electron beams emitted from the electron gun in horizontal and vertical directions with respect to the screen; and a cathode ray tube including a shadow mask aligned from the screen at a predetermined distance, characterized in that the electron gun includes a triode section having a cathode for generating electron beams, a grid electrode aligned to be adjacent to the cathode and an accelerating electrode aligned to be adjacent to the grid electrode, a variable voltage being applied to the grid electrode or accelerating electrode.
The electron gun of the color image display device according to the present invention preferably includes: a triode having a cathode for generating electron beams in accordance with inputted video signal voltage, a grid electrode aligned to be adjacent to the cathode, and an accelerating electrode aligned to be adjacent to the grid electrode.
The color image display device further comprises a brightness compensating device for applying a brightness compensating voltage for compensating a brightness difference in accordance with thickness of the panel glass to the grid electrode or the accelerating electrode, the brightness compensating voltage being variable depending on the beam current emitted from the cathode.